This invention concerns a spherical ball valve comprising a valve body defining an upstream flow tube and a downstream flow tube, a spherical valve plug or ball with a calibrated opening, means for rotating the ball to turn it from a closed position to an open position and vice versa, and sealing means to ensure fluid-tight closure of the valve with the ball in closed position.
The prior art throttling-type ball valves comprise a sealing ring which is permanently engaged with the ball regardless of the position of the ball member. This seal is therefore continuously stressed and tends to have a limited life. Besides, engagement of the seal with the ball also adversely affects the accuracy of the valve.